The thermal ink jet process applies a dilute aqueous ink onto the surface of a paper by heating a small volume of the ink in a small chamber with an orifice that is directed at the recording paper. The small volume of ink that is heated rapidly reaches its boiling point, and the steam bubble formed propels a tiny drop of liquid ink at the paper, where the drop produces a single dot in a dot matrix that forms a character or image on the sheet. This process requires an ink that is low in solids and high boiling components so that it is capable of boiling rapidly without leaving a residue that can foul the heating element, and clog the orifice. Therefore, up to 96 percent of ink jet printer ink is a mixture of water and low molecular weight glycols. Although such an ink boils quickly when heated to ensure rapid printing, and is not prone to clog, it results in an applied ink that is very mobile and slow to dry. Therefore, good print quality can be obtained only if the ink colorant or dye remains on or near the outer surface of the paper, and does not spread or move from the point at which it was applied.
It is also important that drying occurs rapidly to prevent smearing of the colorant. In printers that are not equipped with heating elements, the water and glycol components of the ink must penetrate into the body of the paper for proper drying of the colorant on the surface. If the colored phase is carried into the paper with the liquid phase as it penetrates into the paper, or if the colorant migrates across the surface of the paper, the quality of the resulting print or image will be poor. Also, dry ink colorant that is not permanently fixed on the paper will blot or run if the printed surface becomes wet or is marked with a highlighter. Therefore, the dry ink should have excellent water and highlighter fastness properties for optimum performance.
In most applications, multipurpose office papers provide inadequate or poor thermal ink jet print quality. This is particularly true where multicolor printing with concomitant superimposed ink applications is utilized. The poor print quality is compounded in printers that apply the colors in one order when the print head moves to the right and the reverse order when the print head moves to the left. Multipurpose office papers often allow the colorant to penetrate into the paper, which results in reduced optical density of the printed image, and increased show through on the reverse side of the paper. Multipurpose office papers that are highly sized prevent liquid penetration, leading to higher ink optical density, but, also, excessive feathering and spreading.
One method of improving thermal ink jet print quality is to apply a material to the paper surface that binds the ink colorant to the surface, but allows the water/glycol liquid phase to pass into the body of the paper, which speeds drying. However, the ink colorant often is an unsaturated or aromatic organic compound, and if the surface material interacts too strongly with the colorant the color of the ink can change. Therefore, a surface material must be obtained that prevents the ink colorant from penetrating the paper, but does not interact so strongly as to effect the colorant, and cause a color change.
Oshima et al., U.S. Pat. No. 4,478,910, discloses a paper base stock that is coated with a high specific surface area colloidal silica pigment and a polyvinyl alcohol binder, where the specific surface area of the colloidal silica is greater than 200 m.sup.2 / g.
Miyamoto, U.S. Pat. No. 4,576,867, and Kojima et al., U.S. Pat. No. 4,830,911, teach that application of a cationic quaternary ammonium polymer to a paper stock will improve the water fastness of the resulting paper. Application of such a quaternary ammonium polymer in a size press is also disclosed by Malhotra, U.S. Pat. No. 5,223,338. However, high quality, commercial thermal ink jet recording paper is produced with off machine coaters, i.e., the paper is not treated in the size press of the paper making machine, but must be removed and coated in an additional step using a separate coating process.
Precipitated calcium carbonate ("PCC") has been disclosed as a useful pigment or filler in ink jet paper, but is typically included in a long list of compounds that also includes natural ground calcium carbonate, a form of the compound that differs greatly from PCC in its physical and chemical properties; e.g., natural ground calcium carbonate particles are far larger than those found in PCC, and are normally anionic, whereas PCC is naturally cationic.
Kondo et al., U.S. Pat. No. 5,320,897, describes coating ink jet recording paper with off-the-shelf pigments and a water soluble binder. The pigments which are disclosed as being useful have an apparent specific gravity in the range of 0.10 to 0.50 g/cm.sup.3, and produce a paper with a water contact angle of 45.degree. to 100.degree..
Tsukisaka et al., U.S. Pat. No. 5,007,964, disclose a generic precipitation method incorporating a chelating agent to obtain a porous agglomeration of chain like PCC particles with a BET specific surface area in the range of 25 to 55 m.sup.2 /g for use in a recording paper. Particles with a BET specific surface area of greater than 55 m.sup.2 /g, are disclosed as being undesirable because they have a higher density, are not porous, and exhibit a low ability to absorb oil and water. It is further disclosed that PCC can be used as a paste, or as a powder obtained by pulverizing the dried paste, which is disclosed as not affecting the agglomeration of the particles.
Kunesh et al., U.S. Pat. No. 5,215,734, disclose an accelerated heat aging process for PCC that produces crystals with a specific surface area of about 3 to about 15 m.sup.2 /g and an average discrete particle size of about 0.2 to about 0.9 .mu.m. The PCC is used to improve the optical properties of paper.
In view of the above, there is nothing in the prior art to suggest how to improve the print quality of ink jet recording paper. The present invention provides one such solution to this problem.